Introducing an effective new ship rudder type, Telescopic Rudder

S. E. GHAZI-ASGARA , H. ZERAATGARB AND M. GHIASIC

a Msc graduate of Amirkabir University of Technology, Iran Shipbuilding and Offshore Industrial Complex Co. bAssistant Professor Amirkabir University of Technology Faculty of Marine Technology cAssistant Professor Amirkabir University of Technology Faculty of Marine Technology

Rudder is the most important controlling device in ship maneuvering. Usually, it is a passive equipment of a ship, which generates the control forces and moments by inflow velocity and loses its performance when the ship speed reduces. Many parameters are involved in rudder design such as rudder's shape, area and span. By increasing rudder span, the aspect ratio, rudder area and rudder forces are increased. In addition, research shows that the increasing of rudder aspect ratio is the best option to receive highest performance of rudder than any other change of rudder parameters. Having known that the rudder dimensions limited by ship stern form, rudder span is limited to the ship overall geometry and the stern geometry.

In this research, a new and innovative ship rudder type (Telescopic Rudder) is suggested that allows enlarging the rudder height hence increasing the rudder aspect ratio. This improves the controllability (maneuverability) of vessel by increasing the hydrodynamic coefficients of rudder.

The telescopic rudder made by two pieces, which secondary part slides into the main part. A hydraulic system is used to facilitate sliding thus increasing the rudder height, increasing the aspect ratio and area of rudder producing larger forces. A computer code is also developed to investigate the hydrodynamic characteristics of telescopic rudder by boundary element method (3D-panel). The effectiveness of the new rudder type is studied by the said code in comparison with the conventional type.

Key word: telescopic rudder, hydrodynamic force, maneuverability, aspect ratio.

1. Introduction

Nowadays, considering the increasing number of ships and shipping lines in merchant ship market and also the navy ships, necessity of good maneuvering characteristics is obvious. Consequently, a large variety of control devices of ship motion and maneuvering have been presented.

Rudder is the most important part of ship maneuvering system; so about 15 different types of rudder are suggested till now for various vessel types with different operation and efficiency. In this paper, we will introduce a new type of ship rudder with improved hydrodynamic force and ship maneuverability.

A control surface has one sole function to perform in meeting its purpose and that is to develop a control forced in consequence of its orientation and movement relative to the water [1]. The control force produced by a rudder at the stern of a vessel creates

a moment, RN which causes the ship to rotate and orient herself at an angle of attack to the flow. These forces and moments which are created due to this rotation and angle of attack, will determine the maneuvering characteristics of the ship.

2. Rudder

Some parameters such as principal dimensions of the ship, geometry and body lines of the vessel, rudder and other control surfaces, propulsion system containing engine, gearbox, shafting and propeller are the most important parameters in ship maneuvering and steering.. Ship steering means determine the ship location and the direction of her positioning.

Based on this description, the surge, sway and yaw motions are the main movements in the ship maneuvering The rolling motion has minor effect on the maneuvering calculations; but because of generating the roll in turning, this motion has been investigated in maneuvering too.

The equations of motion can be simplified as below in local coordinate system fixed on ship:

0)()( 1 =+ uXuuX uu && Surge 0)()( 1 =+ rYruYvYvY rrvv && && Sway (1)

0)( =+ rNIrNvNvN rzrvv && && Yaw Where

ruu NNNy

uyx

ux

&&&&& ===

=

,...,, By adopting the non-dimensional form of equations of motion and neglecting the

surge equation, (1) becomes as follow:

0)(0)()(

=+=+

rNIrNvNvNrYrYvYvY

rzrvv

rrvv

&&&&

&&

&& (2)

Where

2

2

523 ,,5.0,,,

5.0 VLrr

VrLr

LII

VLvv

Vvv

Lz

z&&&& ====== (3)

In equation (1) and (2) must included the effect of ships rudder held at zero degree. On the other hand, if we want to consider the path of ship with controls working, the equation of motion must include terms of the right-hand side expressing the control forces and moments created by rudder deflection as a function of time [1].

The linearized component of the force created by rudder deflection acting at the center of gravity of the ship is RY and the linearized component of the moment created by rudder deflection about the z-axis of the ship is RN where R is the

rudder deflection angle and NY , are the linearized derivatives of Y and N with respect to rudder deflection angle R [1].

With these assumptions and some simplifications, the equations of motion including the rudder force and moment are as follow:

==

==

22

)(

vy

zrzz

Rrvz

Rrvy

YINIn

NrNvNrnYrYvYv

&&

(4)

The simplest and most common type of control surface is the all-movable rudder. Chord dimension parallel to the direction of motion, span dimensions normal to the direction of motion and thickness dimension normal to both the span and the chord. The mean value of chord is c and the mean span b is the average of the span of

leading and trailing edges of the rudder. The ratio cb= is the geometric aspect ratio

and the profile area rA may be taken as cb [1].

Figure.1 Typical Rudder Shape

Consider a rudder as a separated body which fully immersed in an inviscid fluid

and a uniform flow which unaffected by ship hull and propeller hits the rudder by angle of attack . The combination of forward velocity and angle of attack will induced a circulation around the rudder which produces a lift force on the rudder. Since the flow is considered in the steady state, two dimensional, ideal, non-viscous and deeply submerged, there is no drag force and the total force due to the angle of attack will act normal to the direction of free-stream velocity. However, because rudder has a finite aspect ratio, two dimensional theory dose not accurately predict the

acting forces. When the rudder is at an angle of attack, vortices are shed over the root and tip of the rudder, which induce velocities in the plane determined by the span and thickness. These velocities when added to the stream velocity cause an induced drag force in the direction of motion [1].

The total resultant hydrodynamic force in a real fluid arising from the effects described in above is shown in figure.2 as acting at a single point called the center of pressure. This force may be variously resolved into any number of components. The three components which involve in ship control are a lift (L), normal to the direction of motion; drag (D), parallel to the direction of motion, and a y-component normal to the axis of the ship. This latest component is the reason for having rudder. If there were no interaction between the pressure field around the rudder and the adjacent ship and its appendages, this y-component would be the control force RY and the moment of this component about the z-axis of the ship would be the control moment RN .

RRrudder

RrudderrudderR

RRrudderR

DLXxYNN

DLYY

cossin.

)sincos(

===

+== (5)

Where the drift angle at the rudder is R , Rx is the distance from the origin of the ship to the C.P. of the rudder [1].

Figure.2 Rudder Force

Whose component of the total rudder force which introduced, F is of importance

in rudder design and the production of this component times the distance of the center of pressure from the centerline of the rudder stock yields the hydrodynamic torque experienced by the stock. The rudder effectiveness in maneuvering is mainly determined by the maximum transverse force acting on the rudder. Rudder effectiveness can be improved by:

- Rudder arrangement in the propeller slipstream.

- Increasing the rudder area. - Better rudder type. - Shorter rudder steering time [2].

There are various kind of rudder developed over the years.

Figure.3 Rudder Type [2]

- Rudder with hell bearing (simplex) - Spade rudder - Semi-balanced rudder - Flap rudder - Rudder with rotating cylinders - Active rudder/rudder propeller - Voith schnider - Speed z-drive - Kitchen rudder - Jet flap rudder [2]

And now we present the new one type called telescopic rudder.

3. Primary plan of a new rudder (Telescopic Rudder)

In present study a new rudder type is developed which provides the arbitrarily increasing of rudder height, so a more effective rudder will be achieved. Because of restricted aspect ratio, there is no similar flow pattern in parallel level with rudder section area, normal to its vertical axis. In addition in both ends of rudder, three dimensional cross flows will be created which by decreasing of the aspect ratio; these cross flows will increase and lead to reduce of the generated lift force by rudder in various angles of attack.

Whatever the rudders root will close to the hull, the creation of cross flows in this part will be avoided; thus the lift coefficient of rudder will increase. Also because of small distance between rudder tip and the outward flow from the propeller, the cross flows are strongly created in this region (rudder tip) which causes to generate flow in-plane of rudder and reduce the lift coefficient. In addition there are many factors involved in rudder design. The most important parameters are rudder shape, area and height.

Rudder must be located in the limited region by the stern geometry. This restricts the rudder dimensions. The telescopic rudder idea makes it possible to loose these limitations during free navigation and achieve more effective rudder.

Increasing the rudder height has a direct influence on some parameters such as aspect ratio, rudder area and decreasing the 3-d cross flow over the rudder root and tip. In addition increasing the stability of ships with high block coefficient, more control on rolling angle and having a more efficient rudder even at low speed near the harbor are some benefits of increasing the rudder height.

The telescopic feature makes it possible to save the previous gains and in necessary conditions such as crossing the shallow channels or in docking condition, comply with shape of ship restrictions and brings out the rudder to initial shape.

4. Structure of telescopic rudder

The telescopic rudder design consists of two parts which the smaller rudder is located inside of the main one and has the ability to slide through it (open/close). The main part is similar to a conventional rudder and in close situation (when the smaller part locates inside the main part) it is exactly a conventional rudder.

The second part which is located inside the main part structure (mother), is smaller and in certain time it can slide out by a control system (hydraulic control) and increase the rudder span; therefore the aspect ratio of rudder will be increased due to increased rudder span and regarding enlargement of rudder area, the greater force and torque will be created.

The telescopic rudder design is more suitable for a large vessel with single passive rudder and propeller. At occurring an accident such as ship collision, by using the telescopic rudder escape from the dangerous area in shorter time and space can be done.

In addition this rudder can help faster maneuvering in harbor area if the water depth would be suitable for its usage. Also this feature is more useful for various sizes of naval vessels during missions. It can provide more powerful maneuvering according to ship speed and it is suitable for track of a target faster and escape from torpedo. Regarding the structural considerations, rudder should be stiffened with vertical and horizontal webs. For installation of the sliding part in main rudder, the arrangement of stringer plate should be in a manner state that in addition to the strength of the main rudder, the required space for moving of the sliding part can be provided. Worthy of mention, this feature is capable to done for various kind of rudder type such as all-movable or horn rudder, etc.

There are some structural configuration and manner of telescopic rudder operation shown in below figures.

Figure.4 Telescopic Rudder - Model A

Figure.5 Telescopic Rudder - Model B

Figure.6 Telescopic Rudder (horn) - Model C

Just as seen in model A, by increases the number of horizontal web plate instead of

vertical and hollowed one, the structural strength of rudder have been provided. Model B stiffened as usual however because of hollow stringer the number of stiffening web increased. Model C is a simple prototype of horn rudder which can be stiffened similarly as both previous models.

There are several challenges in field of machinery and equipment in telescopic rudder which mentioned briefly as following:

- Designing of the control system for rudder movement. - Designing of the steering gear and bearing for rudder. - Designing the proper structure to achieve an acceptable requirement. - Sealing the rudder containing design a seal or sealing system (e.g. using

compress air system to prevent water influence during slide telescopic part). - Examine the fouling of sliding part and its effect on seal system.

5. Mathematical model for hydrodynamics aspects

Potential theory is an extremely well developed and elegant mathematical theory, devoted to the solution of Laplaces Equation:

02 = (6) There are several ways to view the solution of this equation. The one most familiar

to aerodynamicists is the notion of singularities. These are algebraic functions which satisfy Laplaces equation, and can be combined to construct flowfields.

The most familiar singularities are the point source, doublet and vortex. We can write the expression for the potential at any point P as

=0

.)1(141)(

S

dsnrr

p (7)

The value of at any point P is now known as a function of and n

on the

boundary. By using filaments vortex for distribution of singularities along surface we can determine the velocity field from the Biot Savart law as below:

( ) ( ) =

c rrrsdrrrV 3

1

11 )(4 rr

rrrr

(8)

Figure.7 Filament vortex and flow field

6. Relation between aspect ratio and hydrodynamic coefficient

In present numerical modeling, we use below model for study aspect ratio and hydrodynamic coefficient relationship.

Fig.8 - NACA 0018, 100x20 mesh size

Gain result shown in figure.9

Fig.9 Relation between Lift & Drag coefficient with and

Conform to suppose, by increasing the aspect ratio the lift and drag coefficient will

be increased. In figure.10 the changing location of center of pressure with increasing aspect ration has been shown.

Fig.10 Relation between XR with and

Regarding this figure we can find that when increase the position changing in CP will decrease. Therefore the hydrodynamic torque acting on rudder stock will have a brief changes, which is appropriate for design of rudder shaft. This is a one of the other advantage of using telescopic rudder.

7. Effect of enlarging rudder span on hydrodynamic coefficient

According to above obtained results, we investigate the enlarging rudder span on lift and drag coefficient now. We consider below models:

Rudder with NACA 0018, span height 3=h , chord length 1=C , angle of attack o15= , taper ratio = 1.3 , telescopic chord length C75.0 , leading edge offset 0.1.

Fig.11 Rudder Geometry Data

Mesh size table as follow, where n is the number of panel on rudder circumference,

m is the total number of panel along span (include main and sliding part).

Table1. Mesh size

hh

][ 1mmn type

0 119x54 1 0.05 119x50 [42] 2 0.10 119x53 [43] 3 0.15 119x55 [43] 4 0.20 119x55 [41] 5 0.25 119x59 [43] 6 0.30 119x61 [43] 7 0.35 119x59 [40] 8 0.40 119x61 [40] 9 0.45 119x61 [38] 10 0.50 119x59 [36] 11 0.55 119x61 [36] 12 0.60 119x61 [34] 13

Fig.12 Telescopic Rudder Geometry

As it expected by increasing rudder span the hydrodynamic rudder force increased.

Fig.13 Lift & Drag Coefficient Vs Increasing height

Regarding figure.13 there is linear relation between increasing rudder span with

hydrodynamic coefficients. If the rudder height increases 0.6 time to initial height the lift and drag coefficient will increase 1.46 and 1.34 times respectively and improve the maneuvering characteristics.

The figure 14 and 15 show the contour of pressure coefficient on rudder and the stream lines in model 13.

Fig. 14 Contour of Pressure Coefficient

Fig. 15 Streamline on telescopic rudder

8. Conclusions and closing remarks

There are linear relationship between enlarging rudder span with lift and drag coefficient.

Position changing of CP in high span rudder is less than low span. Therefore the hydrodynamic torque acting on rudder stock will have a brief changes, which is appropriate for design of rudder shaft.

Increasing the rudder height has a direct influence on aspect ratio, rudder area and decreasing the 3-d cross flow over the rudder root and tip. Therefore we observed increasing lift coefficient value.

Because of discontinuity in connection of main to telescopic part of rudder, vortex flows have been created in downstream flow.

Increasing the lift and drag coefficient will increase the lift and drag force consequently maneuvering characteristic of vessel will improved.

9. Nomenclature

x , y , z system of reference axis Potential function u , v velocity components

&=r Angular velocity u& , v& , && acceleration components Displacement

zI Mass moment of inertia X ,Y resultant total force N Resultant total moment

uX , uX & derivative of X with u , u& 1u Initial value of u

vY , vY& , rY , rY& derivative of Y with v , v& , r , r&

11 uuu = vN , vN & , rN , rN & derivative of N with

v , v& , r , r&

Y Derivative of Y with R Rudder deflection angle N Derivative of N with

b Mean span c Mean chord

Rr AA , Rudder area , Aspect ratio

Angle of attack L Lift force D Drag force

rudderY Rudder control force

rudderN Rudder control moment

R Drift angle Rx Distance from the origin

of the ship to the C.P. of the rudder

References

[1] Edward V. Lewis, Editor. PRINCIPLES OF NAVAL ARCHITECTURE. Vol. III Second Edition, Pub. The Society of Naval Architects and Marine Engineers, Jersey City, NJ. 1989.

[2] Volker Bertram. Practical Ship Hydrodynamics. Pub. Butterworth-Heinemann. First published 2000.

[3] W. H. Mason. APPLIED COMPUTATIONAL AERODYNAMICS. Professor of Aerospace & Ocean Engineering Virginia Polytechnic Institute & State University, 1998.

[4] William Devenport. Wings in Ideal Flow & Non-Lifting 3D Flow. Virginia Tech Department of Aerospace and Ocean Engineering. 2005

Nowy efektywny ster okrtowy ster teleskopowy

Ster jest najwaniejszym urzdzeniem kontrolujcym w trakcie manewrowania statkiem.

Zazwyczaj jest pasywnym elementem wyposaenia statku, ktry wytwarza si i moment dziki napywowi wody, i traci sprawno, gdy prdko statku si zmniejsza. Z projektowaniem steru zwizanych jest wiele parametrw takich jak ksztat, pole powierzchni i rozpito steru. Przy zwikszaniu rozpitoci steru rosn wyduenie, pole powierzchni i siy. Badania pokazuj, e zwikszenie wyduenia steru jest, w porwnaniu ze zmian kadego

innego parametru, najlepszym rozwizaniem, eby uzyska najwysz sprawno steru. Poniewa wymiary steru s ograniczone przez ksztat rufy statku, rozpito steru zaley od rozmiarw statku i rufy.

W trakcie bada opracowano nowy typ steru - ster teleskopowy, ktry pozwala na zwikszenie wysokoci steru, a dziki temu na zwikszenie wyduenia. To rozwizanie pozwala na poprawienie sterownoci statku poprzez zwikszenie wartoci wspczynnikw hydrodynamicznych steru.

Ster teleskopowy skada si z dwch czci. Cz ruchoma przesuwa si wewntrz czci podstawowej. Do przesuwania czci ruchomej zosta zastosowany ukad hydrauliczny. Po wysuniciu czci ruchomej zostaje zwikszona wysoko steru, co powoduje wzrost wyduenia, powierzchni steru i siy hydrodynamicznej. W celu zbadania charakterystyki hydrodynamicznej steru teleskopowego opracowany zosta program komputerowy oparty na metodzie elementw brzegowych (trjwymiarowa metoda panelowa). Przy pomocy wspomnianego programu zbadana zostaa efektywno nowego steru w porwnaniu ze sterem konwencjonalnym.